New, simple and efficient method for the synthesis of imidazo-azines by flash vacuum thermolysis of tert-butylimines of pyrimidine-2-, pyrazine-2-, quinoline-2-, quinoxaline-2- and isoquinoline-1-carbaldehydes

•Flash vacuum pyrolysis of tert-butylimines yields imidazo-azines.•The reactions are investigated by product isolation and online photoelectron spectroscopy.•The reaction mechanisms involve sequential elimination of two methyl radicals.•The reaction mechanisms are supported by DFT calculations.

Flash vacuum thermolysis reactions of N-(tert-butyl)-N-(pyrimidin-2-ylmethylidene)amine (1), N-(tert-butyl)-N-(pyrazin-2-ylmethylidene)amine (2), N-(tert-butyl)-N-(quinolin-2-ylmethylidene)amine (3), N-(tert-butyl)-N-(quinoxalin-2-ylmethylene)amine (4) and N-(tert-butyl)-N-(isoquinolin-2-ylmethylidene)amine (5) have been investigated. The formation of 3-methyl-imidazo[1,5-a]pyrimidine(6), 3-methyl-imidazo[1,5-a]pyrazine (7), 1-methyl-imidazo[1,5-a]quinoline (8), 3-methyl-imidazo[1,5-a] quinoxaline (9) and 3-methyl-imidazo[5,1-a]isoquinoline (10) as reaction products was observed. Excellent yields of imidazoazines from monocyclic imines 1 and 2 were found, whereas from bicyclic imines 3-5 slightly lower yields (50-75%) of the major products and formation of byproducts such as quinolone and isoquinoline were observed. These cyclizations were found to occur fully regioselectively onto the nitrogen atom of the adjacent ring. UV-photoelectron spectroscopy combined with FVT and quantum chemical calculations were applied for direct monitoring and characterization of the thermolysis products. The proposed mechanism of these reactions are substantiated by DFT calculations.

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